1. Field of the Invention
The present invention relates to a code modulating method and a code modulating apparatus, a demodulating method and a demodulating apparatus, and an information recording medium, all of which are used for recording or reproducing digital data on a recording medium, such as an optical disk.
2. Description of the Related Art
On recording data on a recording medium, such as an optical disk, data modulation is carried out so as to be matched with the recording medium. On recordation or reproduction of data on or from the recording medium, DC components included in signals or codes recorded or reproduced are liable to cause fluctuation of a servo control signal of a disk device to occur, or to cause jitters to occur. Therefore, it is preferable that modulation signal or codes include no DC components, if possible.
A DVD (Digital Versatile Disc) uses eight to sixteen ({fraction (8/16)}) modulation which modulates an 8-bit data bit train into a 16-bit data bit train in accordance with a run length limiting RLL (2, 10) rule. Herein, it is to be noted that the RLL (2, 10) converts a data bit train into a channel bit train which includes, between adjacent ones of “1” bits, “0” bits not smaller than two (2) and not greater than ten (10). Specifically, numeral “2” in the RLL (2, 10) rule denotes the minimum number of continuous “0” bits between the adjacent “1” bits (also referred to as a minimum run length). On the other hand, numeral “10” denotes the maximum number of continuous “0” bits between the adjacent “1” bits (also referred to as a maximum run length). Further, the {fraction (8/16)} modulation is helpful to suppress the DC component by selecting a code word from a plurality of tables based on a DSV (Digital Sum Value). The DSV is defined in connection with the channel bit train after NRZI conversion and denotes a total sum of “+1” obtained by successively adding +1 and −1 on occurrence of “1” and “0”, respectively, from a leading bit of the channel bit train. The DSV serves to indicate a level of the DC components included in the channel bit train. In the {fraction (8/16)} modulation, a code word is selected from the plurality of preset tables so that the DSV becomes minimum and, thus, the fluctuation due to the DC components in the channel bit train can be suppressed.
As mentioned above, in the {fraction (8/16)} modulation, the data bit train is separated into each data word of an 8-bit unit, and is converted into a code word of 16 channel bits. An encoding rate indicated by a rate of the data bits to the channel bits is equal to ½. A high encoding rate implies that a long time can be available for detection of each channel bit. As a consequence, the encoding rate is preferably high.
Known in the art is another code that is defined in accordance with an RLL rule for an optical disk or a magnetic disk, that has the encoding rate of ⅔ which is higher than that of the {fraction (8/16)} modulation, and that is obtained by ⅔ modulation. More specifically, in the ⅔ modulation, 2 data bits are modulated into 3 channel bits under an RLL (1, 7) rule in which the run length of “0” is equal to one or more and seven or less. In other words, the minimum run length of “0” is “1” and the maximum run length of “0” is “7”. FIG. 15 shows a code conversion table for the ⅔ modulation in the typical RLL (1, 7). In the code conversion table shown in FIG. 15, 3 channel bits are obtained by referring to 2 data bits as a modulation target together with a succeeding or subsequent data bit and an end bit of precedent channel bits. In FIG. 15, reference symbol “x” is indicated in the subsequent data bit and the precedent channel bit and may have the “0” bit or “1” bit. In the ⅔ modulation based on the RLL (1, 7), the data bit train is converted into the channel bit train containing “0” bits that are not smaller than one and not greater than seven between the adjacent ones of “1” bits. Therefore, in the NRZI recording, a mark or space is limited to 2T or more and 8T or less (herein, reference symbol T denotes a channel bit length). Japanese Unexamined Patent Publication Nos. 10-340543 (patent document 1) and 2000-332613 (patent document 2) suggest a method for encoding the data bit train into the channel bit train under the above-mentioned RLL (1, 7) rule.
Herein, it has been pointed out that, in a reproduction signal recorded with a high density in a disk, detection of the channel bit becomes difficult because a signal amplitude becomes small due to a short pattern of mark and space lengths. A detecting window width standardized by the data bit length is as narrow as ½ in the case of the {fraction (8/16)} modulation and is widened to ⅔ in the case of the (1, 7) modulation. However, the shortest mark length or space length is as long as {fraction (3/2)} in the case of the {fraction (8/16)} modulation and is as short as {fraction (4/3)} in the case of the (1, 7) modulation. Such a reduction in the shortest mark length or space length brings about an amplitude reduction of the reproduction signal recorded with the shortest mark or space in a recording portion and reproduced from the recording portion. The reduction of the amplitude is liable to increase jitters due to noises when a reproduction signal is converted into binary data by the use of a comparator. The above-mentioned problem which results from continuous occurrence of the minimum run length has been pointed out in Japanese Unexamined Patent Application Publication No. 11-346154 (patent document 3).
However, when either the shortest mark length or space length alone is shortened, a reduction in amplitude can be compensated by using PRML (Partial Response Maximum Likelihood) detection so as to process the reproduction signal. In the PRML detection, a reproduction waveform is compared with a candidate waveform obtained from a plurality of candidates of the channel bit trains, and selection is made about the channel bit train which corresponds to the candidate waveform which is most likely to be the reproduction waveform. According to this detection, even when candidate waveforms partially include a pattern such that a signal amplitude is reduced and the signal amplitudes of the remaining candidate waveforms are large, detection errors can be decreased, on the assumption that the former candidate waveforms and the latter candidate waveforms may be distinguished from each other.
The modulating method for the ⅔ modulation defined in accordance with the RLL (1, 7) described in the related art has an excellent feature that the encoding rate is as high as ⅔ and the detecting window width can be widened. However, in the channel bit train under the RLL (1, 7) modulation, the run length of “1” appears with a high probability and, as a result, the shortest pattern of the 2T pattern is liable to continuously occur (where reference symbol T indicates the channel bit length). The above-mentioned shortest pattern corresponds to the shortest mark or the shortest space in the NRZI recording. In the PRML detection, a detection error therefore frequently occurs in the reproduction signal which includes continuous patterns of 2T. Using the PRML detection makes it possible to distinguish the reproduction waveform with the 2T patterns, from the reproduction waveform with the 3T or more patterns. However, in the portion which has the continuous patterns of 2T, it is difficult to specify the phase of the channel bit train only on the basis of the reproduction waveform in the above-mentioned portion.
In the PRML detection, the channel bit train arranged in the portion with the continuous patterns of 2T is specified by referring to information obtained from the reproduction waveform located or arranged before/after each continuous pattern. Mixture of the noise in the reproduction waveform tends to cause a detection error to occur in the case where detection is made in the state of shifting a whole of continuous patterns of 2T by one channel bit length of T in a long continuous portion of 2T. In this event, a detection error is spread over an entire detected range shifted by one channel bit and lasts for a plurality of bytes.
In the reproduction signal having the long continuous patterns of 2T, namely, the minimum run length 1, a low signal amplitude state is continued for a long time. This makes it difficult to extract a channel clock from the reproduction signal. Continuation of the 2T patterns is also not preferable in view of instability in the extraction of the channel clock. Each of the patent documents 1 and 2 discloses the encoding method under the RLL (1, 7) rule. Further, each of the patent documents 1 and 2 discloses a technology that the redundancy is reduced and the DSV is controlled under the RLL rule. However, no teaching is made at all about the problems caused by the continuous state of the minimum run length 1 and its solving means. Furthermore, the patent document 3 discloses that the continuous state of the minimum run length is restricted to a predetermined number of times by using a converting table having a converting section. However, the patent document 3 is directed only to using a variable length code but never considers any encoding rate.
In addition, it is disadvantageous on reproducing, from the disk, the signal recorded by using the (1, 7) modulation a low frequency component is included in the reproduction signal. Further, the low frequency component in the reproduction signal undesirably varies a servo control signal for controlling a disk device.